TUESDAY, April 8, 2014 (HealthDay News) -- Pregnant women may face increased risk for early-term delivery during heat waves, according to a large new study from Canada.

Researchers analyzed data from 300,000 births in Montreal between 1981 and 2010, and also looked at summer temperatures that occurred during those years.

The University of Montreal team found that the risk of early-term birth (37 weeks to 38 weeks' gestation) was 27 percent higher when temperatures were 32 degrees Celsius (about 90 degrees Fahrenheit) or above for four to seven days. The risk was 17 percent higher when it was that hot for three days.

Hot weather had little effect on the risk of preterm birth (less than 37 weeks gestation), according to the study published online recently in the journal Epidemiology.

While the harmful effects of hot weather on seniors are well known, there has been little research into how it affects pregnant women, said study leader Nathalie Auger, of the university's department of social and preventive medicine.

She said that some small studies suggest that hot weather may affect the uterus in ways that can trigger early-term delivery.

The findings are important because early-term babies are at increased risk for health problems, Auger noted.

"Studies have shown that children born at 37 or 38 weeks suffer more respiratory problems compared with children born at term," she said in a university news release. "Early-term newborns are also at greater risk of death."

Although the study found a connection between summer heat waves and higher risk of early-term pregnancy in women, it did not prove a cause-and-effect relationship.